Adjustable electrical resistor



March 12, 1968 w. J. MAIRS 3,3 3,39

ADJUSTABLE ELECTRICAL RESISTOR Filed May 12, 1966 2 Sheets-Sheet 1 /IVI/ INVENTOR WILLIAM J. MAIRS March 12, 1968 w. J. MAIRS 3,373,394

ADJUSTABLE ELECTRICAL RESISTOR Filed May 12, 1966 2 Sheets-Sheet 2 FIG.5

INVENTOR WILLIAM J. MAIRS BY yfl ATTORNEY United States Patent Ofiice 3,373,394 Patented Mar. 12, 1968 3,373,394 ADJUSTABLE ELECTRICAL RESISTOR William J. Mairs, 85 Canterbury Road, Waltham, Mass. 02154 Filed May 12, 1966, Ser. No. 549,516 14 Claims. (Cl. 338-143) ABSTRACT OF THE DISCLOSURE An adjustable electrical resistor comprising a tubular housing, a helical wire resistance element embedded in the inner surface of the housing, end caps closing off both ends of the housing, and first and second terminal leads mounted in the end caps. The first terminal lead is permanently connected to one end of the resistance element. The second terminal lead carries a contact member adapted to moveably engage the resistance element and is mounted so as to be able to advance the contact member along the resistance element by rectilinear and rotational movement.

This invention relates to electrical resistors and more particularly to miniature adjustable precision resistors.

As is well known, precision electronic equipment usually requires the use of electrical resistors with extremely accurate resistance values. To satisfy this need commercial resistor manufacturers produce fixed resistors over a wide range of standard values. Electronics laboratories concerned with the development of precision equipment customarily carry an inventory consisting of many different size resistors to satisfy the needs of the design engineer. However, in many cases, particularly in the bread board design stage, the exact resistance value required to be inserted in a particular portion of a circuit may not be accurately known or, if known, may not be satisfied by any of the available fixed resistors. At such times the design engineer may be forced to combine several resistors to get the correct resistance value or may have no choice but to purchase the needed item at the expense of added cost and loss of time. Moreover, even after a circuit has been designed so as to take advantage of standard resistance values, variation in other components such as transistors, etc. may dictate small resistance charges to achieve the desired level of operation. Although these problems may be reduced by utilizing trimmer potentiometers as variable resistors, it is not a Wholly satisfactory solution because of such factors as cost, lack of precision, insufficient resolution or limited resistance range. This is particularly true in the case of trimmer potentiometers having a metal film resistor element. The latter lacks the stability and uniformity of a wire resistor.

Accordingly the primary object of this invention is to provide a precision electrical wire resistor that is adjustable over a relatively wide range of resistance values, has a resolution which is virtually infinite, and may be locked at a selected resistance value.

A further object is to provide a precision electrical resistor that is adjustable and that can be made in miniature sizes, whereby to satisfy the needs of transistorized circuitry.

Adjustable electrical resistors embodying the present invention consist of a helical wire resistance element, and a housing molded around the resistance element and provided with end closure means for supporting two termminal leads, one of which is connected to one end of the resistance element and the other of which is movable and carries a contact member adapted to engage and move along the resistance element.

Other objects and many of the attendant advantages of the present invention will become more readily apparent from reading the following specification which is to be considered together with the accompanying drawings, wherein:

FIG. 1 is a longitudinal sectional view of a preferred embodiment of the invention on an enlarged scale;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a fragmentary longitudinal sectional view on an enlarged scale through an alternative embodiment of the invention;

FIG. 4 is a side elevation, with certain parts shown in section, of still another embodiment of the invention;

FIG. 5 illustrates the right hand end of the embodiment of FIG. 4 with one lead removed; and

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 4.

Turning now to FIG. 1, the preferred embodiment of the invention comprises a helical resistance element 2 that is made of bare metal wire of predetermined electrical conductivity. The helical resistance element is space wound, that is, adjacent turns are spaced from each other so as to prevent a short circuit from occurring between adjacent turns. The helical resistance element 2 is encased in a cylindrical housing 4 made of a plastic material having satisfactory electrical insulating characteristics. By Way of example, the housing may be molded from an epoxy resin. The housing is molded around the resistance element in such a way that the resistance element is embedded in its inner cylindrical surface 6. The resistance element is only partially embedded in the housing, with the result that each turn thereof has an inner surface portion 8 that projects from the inner surface 6 and is exposed for engagement with a movable contact member described below. The inner portions 8 of all of the turns of the resistance element together define a continuous helical contact surface.

At one end of the housing 4 is fitted with an end closure in the form of a cap 12 that may be made of metal but preferably is made of the same plastic material as the housing. The cap fits over the end of housing 4 and is cemented thereto. At its center the end cap 12 has a hole 14 in which is positioned a terminal lead 16 made of bare conductive metal wire that is still but bendable. This terminal lead projects outwardly from the end cap 12. Embedded on the inner side of the end cap is a small metal disc 18 with a center hole to accommodate terminal lead 16. The latter is welded to disc 18 so as to prevent it from being pulled out of end cap 12. The terminal lead 16 is soldered or welded to the adjacent end of the resistance element 2.

The opposite end of the resistance element is left unconnected and is concealed by a second end closure in the form of a cap 22 having substantially the same configuration and made of the same material as end cap 12. This second end cap also is cemented to the housing. At its center the end cap 22 has a hole 24 in which is secured a sleeve or ferrule 26 that supports and guides a conductive wire rod 28 that functions as a second terminal lead. In the illustrated embodiment, sleeve 26 is made of metal. However, it also may be made of some other material such as plastic. Thus sleeve 26 could be an integral part of end cap 22. The inner diameter of sleeve 26 is such that lead 28 makes a snug fit but is rotatable and also axially slidable therein. The inner end of terminal lead 28 is provided with a contact member identified generally at 30 made of a conductive spring metal. As seen best in FIG. L2, the contact member 30 is S-shaped, having two curved opposite ends 32 and 34 that are diametrically opposed to each other. Terminal lead 28 is connected to the midpoint or center of contact member 30. The contact member is formed so that before insertion into the housing its maximum dimension measured through its midpoint is slightly larger than the internal diameter of the helical resistance element. When it is inserted into the resistance element its ends are forced inwardly and this compressive loading assures that the ends will remain in contact with the resistance element at all times. The end 32 of the contact member makes a direct electrical connection with the resistance element by virtue of its contact with the turns thereof. For those applications where it is critical to minimize the effective contact area, it is preferred to provide the opposite end 34 of the contact member with an insulating covering or shield 36, preferably made of Teflon, to prevent a direct electrical connection between end 34 and the resistance element. Since Teflon has a low coefficient of friction, shield 36 will not unduly resist movement of the contact member along the resistance element. However, the spring action of the contact member provides sufficient bearing pressure to hold the contact member in a given position in the absence of an exteriorly applied axial force or torque.

To the extent just described, it is believed to be apparent that the structure illustrated in FIG. 1 provides a simple adjustable resistor, with leads 16 and 28 available for electrical connections to an exterior circuit. The precise value of the resistor through which current applied via terminal leads 16 and 28 will fiow is determined by the position of the contact member 30 which in turn is determined by movement of terminal lead 28. Hence lead 28 has a dual function--it may act as a terminal and also as means for adjusting the effective resistance value of the resistor. Moving lead 28 axially provides a relatively coarse adjustment of the position of the contact member along the resistance element, while rotational movement of lead 28 provides a finer adjustment. In this connection it is to be noted that lead 28 may be rotated without undergoing any axial movement. This is unlike the case of a multi-turn trimmer potentiometer where the contact member is supported on a lead screw or on a shaft that is adapted for a screwtype motion.

An important aspect of the resistor of FIGS. 1 and 2 is that it is adapted to become a fixed resistor on a temporary or permanent basis. Preferably this is accomplished by providing a diametrically-extending slot 37 in the outer end of sleeve 26. Slot 37 effectively divides the outer end of sleeve 26 into two corresponding halves. This construction facilitates crimping the outer end of sleeve 26 tight around terminal lead 28 so as to lock the latter in a selected position. As an alternative to crimping, a blob of solder may be deposited in slot 37 to provide a rigid connection between the terminal lead 28 and the surrounding sleeve 26. A further alternative is to omit slot 37 and replace it with one or more holes as shown at 38 in FIG. 1 adapted to receive solder for locking the terminal lead to the surrounding sleeve 26. The use of solder is not essential, although it offers the advantage of being easily removable by heating in the event the setting of the contact member is to be changed. Any one of several conventional plastics cements may be used in place of solder.

FIG. 3 shows an alternative embodiment of the invention. Although not shown, the end closures for the housing 4A may be substantially as shown in FIG. 1. The primary differences between the embodiment of FIG. 3 and the embodiment of FIG. 1 are (a) the helical resistance element is made of a selected commercially available resistance wire 2A that is coated with an insulating enamel 39, and (b) the turns of the helix engage each other. Enamel coated resistance wire is commercially available. Winding the resistance element so that it turns are in engagement with each other has the advantage of providing a greater number of turns and hence a greater total resistance in a given length hous- 4 ing. In manufacturing this embodiment the resistance element 2A and the housing 4A are first assembled in the manner shown, with the outer circumferential portions of the turns embedded in the housing. Then the enamel 39 is removed from the inner circumferential portions of the turns, as illustrated at 40. These uncoated portions of the turns together comprise a continuous helical surface running from one end of the resistance element to its opposite end. Thereafter the end caps and terminal leads are attached in the manner illustrated in FIG. 1. In the completed assembly, the enamel-free portions of the resistance element are engaged by the contact member 30. From the foregoing description, it is believed to be apparent that the insulating enamel 39 prevents shorting between adjacent turns, while the inner enamel-free portions 40 permit the contact member 30 to function electrically exactly as in the embodiment of FIG. 1. In this connection it is to be noted that the contact member is made of relatively small diameter material so that it will engage only one and preferably not more than two or three turnsof the resistance element.

FIGS. 4, 5 and 6 illustrate still another embodiment of my invention adapted for use with printed circuits. Like the embodiments of FIGS. 1 and 3, this additional form of the invention consists of a plastic housing 43 in which is embedded a helical resistance element 2B. The latter may consist of bare wire space-wound as in FIG. 1 or enamel insulated wire with the enamel removed from the inner circumferential portions of the turns as in FIG. 3. In this embodiment the housing 4B is fitted with metal end caps 42 and 44 whose outside diameters are smaller than the corresponding dimensions of the housing. Each cap is formed with a circumferentially extending groove 46 in its inner surface adapted to receive the adjacent end of the housing and automatically center the cap. Each cap is cemented to the housing.

The'left-hand end cap 42 carries a terminal lead 48 made of stiff but bendable metal wire. Preferably the inner end of lead 48 protrudes into the housing and is bent back and secured to end cap 42 by soldering, welding or a conductive cement. The adjacent end 50 of resistance element 2B is conductively secured to end cap 42 by similar means. Alternatively, end 50 of the resistance element may be secured directly to the adjacent end of lead 48. As with the corresponding ends of resistance elements 2 and 2A of FIGS. 1 and 3, the opposite end of resistance element 2B is left unconnected and is spaced from the right-hand end cap 44. The latter is formed with a center hole 52 and a centrally located sleeve or ferrule 54 Whose internal diameter is the same as that of hole 52. Slidably and rotatably mounted in sleeve 54 is a conductive wire rod 56 having on its inner end a contact member 58 made of a conductive spring metal. Contact member 58 is integral with rod 56 and is formed by bending the rod. As seen in FIG. 6, contact member 58 is formed in the general shape of the letter G, in contrast to the contact member 30 previously described that is generally S-shaped. However, it is functionally equivalent to (and may be substituted for) contact member 30 since it also has two oppositely curved portions 62 and 64 that are diametrically opposed to each other and that press outwardly against the surrounding resistance element at all times. One of its curved portions 62 may be shielded by an insulating covering 66, while its other portion 64 makes a direct electrical connection with the resistance element. The insulating covering preferably is made of a plasticlike Teflon that has a low coefiicient of friction so that the contact member can be moved easily by manual manipulation of rod 56. The contact member can be locked at a selected setting by cementing or soldering together rod 56 and sleeve 54. The latter also may be slotted as shown at 67 to facilitate crimping it about rod 56.

This alternative form of resistance element functions electrically the same as the device of FIG. 1, except that rod 56 does not function as a terminal lead but only as a means for adjusting the effective value of the resistor. Instead another conductive wire member 68 is provided to act as a terminal lead. Wire member 68 is secured to sleeve 54. As seen in FIG. 5, sleeve 54 is provided with a circumferential groove 70. Wire member 68 is wrapped around sleeve 54 inside of groove 70 and is soldered or welded in place. This form of connection between wire member 68 and sleeve 54 is strong yet simple, and offers the further advantage that the wire member is brought out radially and at a right angle to housing 4B and sleeve 54. The corresponding portion of lead 48 at the opposite end of the resistor also is brought out radially and at a right angle to housing 4B. Hence both terminal leads 48 and 68 may be used to physically secure the resistor to the front or rear side of a printed circuit board 72, as well as electrically coupling the resistor to circuit elements carried by the same board.

It is contemplated that housing 4B may be formed of a variety of plastic materials, some of which may lack sufficient rigidity and resistance to varying environmental conditions. Accordingly it is contemplated that a sleeve 76 preferably formed of metal may be slipped over the housing to provide added rigidity. This sleeve is sized so as to make a tight fit with the housing. It is held in place by means of a potting compound such as an epoxy cement 78 that is applied between it and the end members 42 and 44. To allow room for the cement and also to prevent direct physical contact with sleeve 76, the end members 42 and 44 are made with an outside diameter smaller than that of housing 4B. Because the sleeve 76 and end caps 42 and 44 are potted together, sleeve '76 functions as an hermetic barrier as well as a support member for housing 413.

The invention just described has several advantages. For one thing it has a simple construction and can be made in miniature sizes. Another advantage is that the helical resistance element can be made of a wire of substantially uniform electrical properties from one end to the other. This uniformity of electrical properties, i.e. resistivity, coupled with the fact that the turns are of uniform size, means that the resistance of the overall helical element is uniformly distributed from one end to the other. A further advantage is that the contact member can be moved both axially and circumferentially. The net result is an adjustable resistor with almost infinite resolution. Hence one employing a resistor of this construction can attain almost any desired value within the limits determined by the total resistance of the resistor element. Moreover, once the user has settled on a particular resistance value, i.e. has positioned the contact member to his liking, he can preserve this value by locking the movable wire rod, e.g. lead 28 to the supporting guide sleeve, e.g., 26, the locking being effected by crimping, soldering, or cementing as desired. Should it be desirable or necessary at a later date to establish a different effective resistance value, it is an easy matter to remove the solder or cement or uncrimp the guide sleeve. Thus an engineer involved in design work can use the same resistor over and over again with the unit having one precise value in one installation and different selected values in other installations, so that the inventory of resistors required to meet the needs of a design engineer can be reduced to .a minimum. Because resistance wire is available in different resistivity ranges, the invention lends itself to production of both large and small value resistors. By Way of example, it is possible to make five different size resistors, one offering a resistance range from 0 to 1,000 ohms, another offering resistance values from 1,000 to 10,000 ohms, a third offering values from 10,000 to 100,000 ohms, a fourth offering values from 100,000 to 1 million ohms and the fifth offering values from one to 20 megohms. Because the adjustable contact member is located at the extreme end of the movable wire rod that supports it and can be moved from one end of the resistor to the other end, the effective value of the resistance can be varied from about 0% to about of the total ohmic value of the resistance element.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts specifically described or illustrated, and that Within the scope of the appended claims, it may be practiced otherwise than as specifically described or illustrated.

I claim:

1. An adjustable resistor comprising an elongated tubular self-supporting housing made of insulating material and formed with open ends and a cylindrical inner surface, a conductive wire resistance element in the form of a helix disposed coaxially within said housing, each turn of said helix embedded in said cylindrical inner surface and having an inner portion thereof exposed for engagement by a moveable contact member positioned within said housing, the exposed portions of all said turns forming a continuous helical surface, first closure means closing off one end of said housing, a first conductive terminal lead secured to and projecting exteriorly of said first closure means, said first terminal lead conductively connected to a selected point on said resistance element, second closure means closing off the other end of said housing, a movable contact member in said housing with a. selected section thereof moveably engaging said helical surface, a second conductive lead having one end connected to said contact member and the other end projecting outwardly of said housing through said second closure means, and guide means on said second closure means slidably and rotatably supporting said second conductive lead so that it may be used to advance said contact member along said helical surface by rectilinear and rotational motion relative to said housing.

2. An adjustable resistor as defined by claim 1 wherein said resistance element is a bare wire and is space wound so that successive turns are out of engagement with each other.

3. An adjustable resistor as defined by claim 1 wherein said resistance element is wound so that adjacent turns engage each other, and further wherein each turn of said resistance element has a thin surface coating of insulating enamel preventing a short circuit with adjacent turns, said inner portion of each turn being free of said insulating enamel so as to permit electrically conductive engagement thereof by said contact member.

4. An adjustable resistor as defined by claim 1 wherein the closure means are caps secured to said housing.

5. An adjustable resistor as defined by claim 1 wherein said guide means is a hollow sleeve disposed in coaxial relation with said helix.

6. An adjustable resistor as defined by claim 5 Wherein said sleeve has an opening in its side for receiving a cement to lock it to said second conductive lead.

7. An adjustable resistor as defined by claim 5 wherein said sleeve is slotted to facilitate crimping it into locking engagement with said second conductive lead.

8. An adjustable resistor as defined by claim 1 wherein said second closure means is a cap cemented to said housing, and further wherein said guide means is a sleeve secured in a hole in said cap in coaxial relation with said helix.

9. An adjustable resistor as defined by claim 1 wherein said contact member and said second lead are formed from a single piece of wire, and further wherein said contact member is S-shaped with its two opposite ends in engagement with said helix.

10. An adjustable resistor as defined by claim 9 wherein said selected section of said contact member constitutes one of its two ends.

11. An adjustable resistor as defined by claim 10 Wherein the other end of said contact member is insulated.

12. An adjustable resistor as defined by claim 1 further including an additional Wire lead secured to said guide means and electrically connected thereby to said second conductive lead.

13. An adjustable resistor as defined by claim 12 wherein said guide means is a metal ferrule.

14. An adjustable resistor as defined by claim 1 further including a metal sleeve surrounding said housing and supporting it against fiexure, and means cementing said sleeve to said first and second closure means.

References Cited UNITED STATES PATENTS Hamacher et al. 338-143 Sorbet 338-184 X OBrian 338-184 X Johnson 338-443 Laubenfels 338-143 

